1. Field
The present invention relates to a light emitting diode, and more particularly, to a light emitting diode with improved light extraction efficiency.
2. Discussion of the Background
Since the development of gallium nitride (GaN)-based light emitting diodes (LEDs), the GaN-based LEDs are currently used in a variety of applications such as full-color LED displays, LED traffic lights and white LEDs.
A GaN-based LED is generally formed by growing epitaxial layers on a substrate such as a sapphire substrate, and includes an N-type semiconductor layer, a P-type semiconductor layer and an active layer interposed therebetween. Meanwhile, an N-electrode pad is formed on the N-type semiconductor layer, and a P-electrode pad is formed on the P-type semiconductor layer. The LED is driven by being electrically connected to an external power source through the electrode pads. In this case, current flows from the P-electrode pad to the N-electrode pad via the semiconductor layers.
Since the P-type semiconductor layer generally has a high resistivity, current is not uniformly distributed in the P-type semiconductor layer but concentrated on a portion at which the P-electrode pad is formed. In addition, the current concentratedly flows through an edge of the LED. The current crowding leads to reduction of a light emitting area, and it results in a lower luminous efficiency. In order to solve such a problem, there is used a technique for spreading current by forming a transparent electrode layer with low resistivity on a P-type semiconductor layer. Since current introduced from a P-electrode pad is spread in the transparent electrode layer and then introduced into the P-type semiconductor layer, the light emitting area of an LED can be extended. However, since the transparent electrode layer absorbs light, its thickness is limited, and therefore, there is a limitation in current spreading. Particularly, there is a limitation in current distribution using a transparent electrode layer in a large-sized LED of is about 1 mm2 or more, which is used for high power.
Meanwhile, extensions extending from electrode pads are used to help current spreading in an LED. For example, U.S. Pat. No. 6,650,018 discloses that a plurality of extensions extend in opposite direction to each other from electrode contact portions 117, 127, i.e., electrode pads to enhance the current spreading. Although current can be distributed throughout a wide area of an LED by using the plurality of extensions, there exists the current crowding that current is still concentrated on portions at which the electrode pads are positioned.
Further, as the size of the LED is increased, it is highly likely that a defect will occur in the LED. For example, a defect such as a threading dislocation or pin hole provides a path along which current rapidly flows, and thus disturbs the current spreading.
Meanwhile, a patterned sapphire substrate is generally used to improve the light extraction efficiency of an LED. A pattern on the sapphire substrate scatters or reflects light generated in an active layer, so that the light lost by internal total reflection in the LED is reduced, and accordingly, the light extraction efficiency is improved.
It is expected that the light extraction efficiency will be improved using the pattern on the sapphire substrate. However, since the refractive index of a GaN-based compound semiconductor layer is relatively high, it is still likely that light will be lost by the internal total reflection in the LED.
Moreover, since electrode pads are generally formed of a metallic material which absorbs light, the light propagating toward the electrode pads is absorbed and lost by the electrode pads.
Therefore, efforts for improving the light extraction efficiency are continuously required.